Thinking about it a bit more, you could potentially resolve it with some custom .htaccess rules that redirect visitors to / that are not being referred from local pages to /yourstartpage.html (renaming your index.html as yourstartpage.html) but that would also force any direct visitors through yourstartpage

Anyone wondering why I want to have index.php and index.html sitting together – I have an html holding page up, but still want to have WordPress sitting in the same directory whilst I populate it with content.